There is a collision detection device which detects front and side collisions of a vehicle respectively and then actuates air bags for front and side collisions respectively. In this collision detection device, acceleration data detected in three directions, namely front, right and left directions, by acceleration sensors installed in the longitudinal and lateral directions are individually treated then air bags for front and side collisions are respectively actuated. And there is a further proposed collision detection device of which a utilization range is expanded to an oblique collision and which avoids an inflation of a side air bag at the time of a front collision, wherein acceleration informations in the three directions are composed and treated as a composite vector (Refer to Japanese Laid Open No.H6-55993 and H6-56000).
In more detail, this device detects accelerations Gy and Gx in longitudinal and lateral directions respectively as shown in FIG. 6 and obtains values fy and fx by integrating the accelerations Gy and Gx with respect to time. Then, the values fy and fx are composed as a composite vector f concerning the collision. Concretely, a magnitude of the collision is calculated according to a formula of (fx.sup.2 +fy.sup.2).sup.1/2, and the direction of the collision is calculated according to a formula of .theta.=tan.sup.-1 (fx/fy).
And a threshold value is set for the magnitude The necessities of actuations concerning air bags are judged from a result of a comparison between the threshold value and the magnitude. Further, an air bag to be inflated is selected according to the direction .theta..
However, the actual collision situation is so complicated that not only the magnitude but also the direction of the collision as shown by the above vector f change every moment. For example, we assume that an end of the vector f changes along a chain double-dashed line 40 as shown in the drawing. In this case, a definition of the direction .theta., .theta.' or .theta." depends on a way of setting threshold value 41, or 42. Consequently, there is a problem in which combinations of the magnitudes and the directions of the vector f are so innumerable that the way of setting threshold value becomes complicated.
This invention is made to solve these problems possessed by the conventional art, and the object of the present invention is offering a collision detection device which can actuate suitable passenger safety devices after selecting them from a plurality of passenger safety devices even when not only the magnitude but also the direction concerning acceleration of a collision change every moment.